I think box-sizing is better than box-width/height because of
box-width/box-height override width/height. This will confuse people which do
not know standarts well. First example confusing, but second more clear, to my
mind:
<div style="box-width: 70%; width: 90%"></div>
<div style="box-sizing: border-box; width: 70%"></div>
So selecting a render method 'box-sizing' and set width/height looks more naturally